The present invention relates generally to a method for cleaving a semiconductor device and more particularly to a method for cleaving semiconductor devices having etched facets wherein the position of the cleave may be accurately selected.
In many modern applications, opto-electronic integration requires the use of accurate etching and cleaving techniques for semiconductor materials so as to have precise control of the dimensions of the emitting facet and the position of the cleave relative thereto. Exemplary applications in which such precise positioning of the cleave is necessary includes semiconductor devices in which a laser is integrated with other electrical or optical components.
In many modern semiconductor fabrication techniques, a wafer from which numerous, individual semiconductor devices will be constructed is processed through conventional semiconductor techniques such that a plurality of epitaxial layers are grown or deposited upon the surface of the wafer. Subsequently a mask is designed for the wafer such that deposited photoresist may be, exposed and developed upon selected surfaces of the wafer. Thereinafter, the wafer is exposed to conventional etching techniques. Following the etching, the photoresist may be removed such that the wafer is patterned into a plurality of semiconductor devices; all of which are still connected by the wafer substrate.
For example, should the individual semiconductor devices which have been patterned upon the wafer be semiconductor laser diodes, the etching process would have produced a pair of opposing etched facets positioned at the longitudinal faces of the patterned semiconductor laser diode. It is desirable to perform as many common steps of the semiconductor devices fabrication processes with the individual semiconductor devices still joined upon the wafer substrate so that each individual device was processed simultaneously. An example of such a common processing step which it would be desirable to perform simultaneously to all the patterned semiconductor devices would be the coating of the etched facets with high-reflectivity coatings to maintain appropriate oscillation within the semiconductor laser diode for proper laser operation. By performing a single deposition of such high-reflectivity coating on all patterned semiconductor devices which are still connected upon the wafer substrate, the processing proceeds more rapidly and efficiently than prior art processing techniques whereby each individual semiconductor device patterned upon the surface of the wafer is physically separated from every other semiconductor device and individually coated with facet coatings. Such separation and individual processing of each patterned semiconductor device requires much greater time and thus results in a corresponding decrease in efficiency of production of such semiconductor devices.
Additionally, in order to increase the number of semiconductor devices harvested from a single wafer, it is desirable to position the semiconductor devices as close to one another as possible, however, they must still able to be separated without destroying the function of the semiconductor device itself.
In order to position many individual semiconductor devices upon a single wafer and to perform numerous processing steps upon each patterned semiconductor device simultaneously while such individual semiconductor devices are still connected upon the wafer substrate, it is desirable that there be an accurate method of cleaving the individual semiconductor devices from one another following their common processing steps such that the individual semiconductor devices are not damaged during the cleaving process.
An additional illustration in which accurate cleaving position is desirable may be illustrated with reference to FIG. 1 in which an etched facet is illustrated in side cross sectional view. As the light emitted from the emitting facet of a semiconductor laser diode has a relatively large divergence, the ledge adjacent to the emitting facet must be relatively narrow such that the diverging output light does not strike the surface of the ledge and either be absorbed or reflected at an angle which would disturb the light's far field pattern. Thus, a cleave is necessary to separate the semiconductor laser device at a point such that the ledge is sufficiently narrow to not intercept the emitted light. Typically, the width of the ledge should be less than the distance between the base of the ledge and the quantum well from which the light is emitted. Thus, if the quantum well is positioned one mircometer from the ledge, the ledge should be less than one micrometer in width. A cleave must be very accurately positioned, as a cleave positioned too far from the emitting facet would make the resulting ledge too wide such that is disturbs the output of the laser diode, while if the cleave is positioned too close to the emitting facet, the emitting facet and its coatings may be cleaved off.
There have been several previous cleaving techniques utilized to separate such individual semiconductor devices from a single wafer substrate. One such technique involves scribing between the semiconductor devices and then applying a force along the length of the scribe in a direction perpendicular to the surface of the wafer so as to crack the wafer along the scribe. This skip-scribing technique is not acceptable for making good quality laser facets such as those utilized with optical devices as the scribe damages the facet and leaves a fractured surface, but may be utilized with the electrical devices.
An alternative cleaving technique is to again scribe a mark in the wafer but instead of applying force along the entire length of the scribe mark, applying pressure to one end of the scribed mark so as to crack the wafer at that point and then shifting the force applied so as to urge the crack to travel the length of the scribed mark. By using such a cleaving technique, the position of the cleave cannot accurately be determined prior to the cleaving and should the cleave vary substantially from the mark scribed in the wafer, the individual semiconductor device may be irretrievably damaged by the cleave. Furthermore, such cleaves must follow atomic crystal planes and therefore cannot be accurately registered, in angle, to the photolithographically - defined features of the processed wafer.
An alternative method for precisely positioning cleaves in semiconductor lasers was described in an article entitled, "Etching of Deep Grooves for the Precise Positioning of Cleaves in Semiconductor Layers" by J. E. Bauers, B. R. Hemenway, and D. P. Witt published in Applied Physics Letters, Volume 46, No. 5, page 453-455 on Mar. 1, 1985. The technique described by Bauers, et al. involves the etching of grooves into a semiconductor substrate followed by the application of force along the deep grooves so as to separate the wafer along the groove. The groove was etched using photoelectrochemical etching in which the semiconductor wafer is immersed in a conducting solution with a low etch rate. Subsequently, a bias is applied to the substrate to deplete the surface of electrons and light with a photon energy greater than the substrate bandgap energy is applied to the region of the substrate of the sample to be etched. Thus, a mask must be applied to the substrate in those areas in which no etching and subsequently no groove is desired. Such a process is continued until a very deep groove is produced at which time the wafer is removed from the conducting solution the bias is removed from the substrate, and the light is removed. Subsequently, the wafer may be flexed so as to induce cleaves along the lines of the grooves. While such a cleaving process may produce reliably positioned cleaves such a process also suffers from several deficiencies, including the inefficiencies involved in precisely aligning the mask along the back side or the substrate side of the semiconductor devices, as well as the length of time required to etched such deep grooves in the semiconductor device.
It would be desirable to develop a cleaving method for use with semiconductor devices in which cleaves could be accurately and reliably positioned. It would also be desirable if such a cleaving method could separate numerous, closely positioned semiconductor devices which have been patterned upon a semiconductor wafer following their processing in common.